21 Sep 2007

Finish chapter 4:

Structure, aggregation, consistence, etc.

Photo courtesy of Lynn Betts, USDA NRCS

http://i.treehugger.com/files/th_images/recycled%20materials.jpg

Texture: proportion of Individual particles

Ex: construction supplies

Structure: arrangement of those individual particles

Ex: a house

1.

2.

Four Structural Types

4.

3.

Spheroidal Structure (granular and crumb)

• Rounded aggregates that lie loosely in soil• Non-porous spherical aggregates are called granular

structure• Porous spherical aggregates are called crumb

structure• Associated with surface soils having high organic

matter content • Most desirable structure because this type of

structure allows for the greatest fluid and gaseous exchange

• Maintenance of this structural type in surface soils should be a goal in agriculture

Spheroidal Structure (granular and crumb)

Soil Aggregate models

Candy or chocolate haystack Computer-generated picture

Platelike (platy) – horizontal plate or lens shape

Generally a subsurface structural type that can be inherited from parent material or created by impact and traffic, particularly in clayey soils

Platelike structure is undesirable because fluid movement is slowed down

Blocklike (angular blocky and subangular blocky) - squarish aggregates with sharp corners (blocky) or round corners (angular)

Blocky is generally a subsurface structural type and more desirable (particularly subangular blocky) for fluid exchange than prismlike or platelike structure

Prismlike (columnar and prismatic)• Vertical columns or pillars in the subsurface• Columnar structure has rounded tops and prismatic

structure has angular or flat tops– found in sodium affected soils (sodic soils)

• Prismlike structure is common in heavier soils • Fluids tend to quickly move down quickly but have

slow penetration laterally into the structure

http://cru.cahe.wsu.edu/CEPublications/eb1633/fig3.gif

Other models that demonstrate soil organic matter effects

• Trail mix bars or any cookie that has a mixture of different size ingredients and preferably coated. – The cohesive nature of

these desserts shows how soil can form stable peds.

Structural units form in response to physical, chemical, and biological processes of weathering:

• Clumping,

• Cementing– Carbonates, Fe oxides, organic secretions, gels,…

• cation bridging,

• roots and fungal hyphae networks,

Aggregation

Aggregate Stability

• If aggregates are not stable, they quickly fall apart when disturbed by mechanical manipulation or water

• If this happens, then soil loses aggregation and fluid movement is greatly reduced (soil seals up)

• Stable aggregates resist disturbance or breakdown • Maintenance of structural stability is critical for fluid

exchange properties of soil• Tillage, organic matter additions and the degradation of the

organic matter are keys to maintenance or improvement of structural stability

A soil aggregate high in organic matter will stick together The water will be clearer (left)

A soil aggregate low in organic matter will slake rapidly. The water will soon be cloudy (right)

Aggregate stability is particularly important on the surface to prevent crusting

When aggregation is lost, pore spaces are filled with clay and silt particles and the soil forms a crust

This problem is commonly seen in the low organic matter soils in the Mesilla Valley

Soilcrusting

Soil Tilth

• A poorly defined term meant to convey the general physical condition of the soil

• A soil of good tilth is physically conducive to plant growth (good aeration, holds water, etc.)

• A soil of poor tilth is not physically conducive to plant growth. It may be cloddy, crusty, hard to work, etc

Soil on the right has better tilth. Higher organic matter results in better and more stable aggregation that results in a more favorable root environment.

Soil Consistence

• Resistance to deformation or rupture under stress by dry or moist soil clods or peds.

• Loose, Friable, Firm and extremely firm when moist;• Loose, Soft, Hard, and up to extremely hard when dry. • Friable is generally the consistency sought after by farmers (it

works easily, clods break apart, but still support roots). • Friability depends much on texture, structure, and ultimately,

organic matter content.

Loose: You have trouble picking out a single ped and the structure falls apart before you handle it

Friable: The ped breaks with a small amount of pressure

Firm: The ped breaks when you apply a good amount of pressure and dents your fingers before it breaks.

Extremely Firm: The ped can't be crushed with your

fingers (you need a hammer!).

• Plasticity is the degree to which soil can be deformed without rupturing

• Stickiness is the capacity of soil to adhere to other objects (like your shoes when walking through a wet field)

• Swelling and shrinking is the change in volume with increases/decreases in soil moisture

How muchstress can we take before we break???